1. Field of the Invention
The present invention relates to a left and right wheels drive system which can transmit drive torque from two drive sources to left and right drive wheels while a drive torque difference is introduced therebetween, and more particularly to a left and right wheels drive system which is suitable for use in an electric vehicle which employs an electric motor as a drive source.
2. Description of the Related Art
In electric vehicles (also, referred to simply as vehicles), there is a related-art technique in which electric motors (motors) are provided for left and right drive wheels, respectively, and a drive torque difference is introduced between the left and right drive wheels as required by controlling the electric motors independently, whereby the turning moment of the vehicle is controlled (for example, refer to JP-A-8-19110 and JP-A-2005-138824).
However, in a running scene in which the vehicle requires a large total drive torque (a total of the drive torque of the left and right drive wheels) as when the vehicle is accelerated, the drive torque of each of the left and right drive wheels has to be increased, and therefore, it is difficult to apply a large drive torque difference between the left and right drive wheels. Namely, in order to apply the large drive torque difference between the left and right drive wheels, the drive torque of either of the motors has to be decreased. However, when this occurs, the total drive torque of the vehicle is decreased accordingly, and the running performance of the vehicle in acceleration, for example, becomes insufficient.
To improve this problem, as is described in JP-A-2006-117011, a related-art technique is considered in which the turning moment of a vehicle is controlled without decreasing the drive torque of both of left and right drive wheels by providing a steering unit for turning the drive wheels by making use of a torque reaction force of motors. However, this related-art technique calls for complexity and an increase in weight of the system concerned.
On the other hand, JP-A-2008-215519 discloses a related-art technique in which a torque shift can be made between two motors by interposing two differentials each employing a planetary gear mechanism and two reduction gear trains each made up of a combination of gears between output shafts of the two motors. As in this related-art technique, in the event that the torque shift can be made between two motors, a large drive torque difference can be introduced between left and right drive wheels while suppressing the reduction in total drive torque.
As has been described above, according to the related-art technique disclosed in JP-A-8-19110, the large drive torque difference can be introduced between the left and right drive wheels while suppressing the reduction in total drive torque by making use of the torque shift between the two motors. However, it has been desired that a system configuration is developed which can produce a desired drive torque difference between left and right drive wheels while suppressing a reduction in total drive torque with a simpler configuration.